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I Why do bubbles of air in water move up?

  1. Sep 28, 2018 #1
    What I am confused about is why do bubbles of air in water move up. I understand why solids and liquids would move up in water if they are less dense. I get the idea that the deeper you go in water the more pressure there is because of the more water weighing down on the water and so there would be a pressure difference between the top and bottom of any object in water and because pressure * area is force there would be a force difference and this would cause a net upwards force and if this net upwards force is larger than gravity than he object would move up. I get how this mechanism works. My problem is with air specifically.

    What I imagine should happen is the water should collapse into the air. The way I imagine it is that air has lots of molecules freely moving about but to me this should allow water to because of gravity and pressure move into the air where the air molecule isn't present and eventually collapse into it. Even if the air molecules are fast enough and random enough that they are able to prevent such a collapse from immedietly happening I do not see how it would move up. As the force exerted on the air molecules in the bottom shouldn't be translated to the air molecules at the top. Because to my knowledge it is hard for two air molecules to hit each other. so the air should simply be squished in until it does disolve rather than moving upwards. I just can't understand how air can displace water a much denser substance to move upwards. Furthermore the water and the air should be around the same temperature. While some other things go into temperature as well other than speed it shouldn't be so easy for air to keep striking the water molecules away to prevent them from collapsing into the air.
    I tried my best in expressing what I found confusing but it might not have been very clear. if anyone wants me to I can definitely elaborate. Thanks for anyone's help on this.
     
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  3. Sep 28, 2018 #2

    phinds

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    I don't get it. You understand how an extremely light Styrofoam ball, for example, would move up but you don't understand how a region of compressed air with exactly the same density would move up? Your issues about the motion of the air molecules are a red herring to your understanding. The only thing that matters is the weight/density of one region relative to the other.
     
  4. Sep 28, 2018 #3

    Nugatory

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    Try thinking about it as "Why does the water move downwards?" instead of "Why does the bubble move up?", and it will be easier to understand why the bubble rises.

    The rest of your question is about how a bubble can persist as a sort of hole in the water. To see how this works you want to be thinking about the difference between a gas and a liquid, and especially how their volume behaves when the pressure changes.

    It's surprisingly difficult to analyze this problem in terms of the behavior of individual molecules. First you understand how the molecules behave in such a way that one substance is a gas and the other a liquid, then you have to understand in detail what happens at the air-water boundary, and only then can you completely explain the rising bubbles. In practice no one is going to do that - we already know that water is a liquid and air is gas, so we take that as the starting point.
     
  5. Sep 28, 2018 #4

    russ_watters

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    They do. Large bubbles break apart into smaller bubbles because of water forcing its way into the bubble. This has nothing to do with why the bubbles rise, though. In fact, it will make them rise faster by reducing the resistance to their rising (reducing the resistance to the water falling).
    Forget about a bubble in water for a second. A volume of gas - any volume of gas - has the pressure that it has because the molecules collide. Right?
    Well, how about looking at it from the other direction: can you understand why water would fall down if there is nothing below it to support it? Should rain not fall to Earth?
     
  6. Sep 28, 2018 #5

    jbriggs444

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    Air molecules are fast enough and there are enough of them so that they prevent the collapse. They effectively "bounce off" the surface of the water. Enough of them are continuously bouncing off to maintain enough pressure to prevent collapse. If a partial collapse does occur, that just squeezes the gas, increasing the number of molecules per unit volume and making them move faster yet.

    Most fluids have cohesion. Fluid molecules "like" to be next to one another. This manifests as surface tension. If a tendril of water were to start pushing its way into a gas bubble, surface tension would pull it back into the bulk of the water. That is why gas bubbles are normally spherical.
     
  7. Sep 28, 2018 #6
    This was really helpful I didn't think of the surface tension aspect. but why does the air bubble move up. I mean do the air molecules bounce off the bottom with greater force to hit the top?
     
  8. Sep 28, 2018 #7

    jbriggs444

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    As has been suggested a couple of times already, think about what the water is doing. The air has nearly equal pressure at top and bottom. The water has greater pressure at the bottom of the bubble. It'll push the bulk of the air upward and flow into the resulting space. The water has lower pressure at the top of the bubble. It'll be pushed away and flow down around the bubble under the influence of gravity.
     
    Last edited: Sep 28, 2018
  9. Sep 28, 2018 #8

    sophiecentaur

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    If you have bubbles of CO2 under a great depth or under pressure in a cylinder (Sodastream etc.), they can dissolve in the water and the bubbles can disappear. That's because of the high solubility of CO2. Nitrogen and Oxygen are not as soluble under the sort of pressure you get in the sea so the air bubbles will be compressed until the inside pressure is the same as the hydrostatic pressure in the water at that level. They will just float upwards as already discussed here.
     
  10. Sep 28, 2018 #9

    russ_watters

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    Yes.
     
  11. Sep 28, 2018 #10

    Mister T

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    Archimedes. The bubble weighs less than an equal volume of water.

    I don't understand what it means for gravity to move. Pressure, either.

    The air molecules inside the bubble are moving around, colliding with the membrane that forms the boundary of the bubble. Likewise for the water molecules on the other side of the membrane. It's these collisions that give rise to what we call pressure. When the pressure inside the bubble equals the pressure outside, the bubble's size stabilizes.
     
  12. Sep 29, 2018 #11
    You can also think of it in terms of energy changes. Systems tend towards minimum PE so the water moves down resulting in the bubbles moving up.
     
  13. Sep 29, 2018 #12

    rcgldr

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    On a side note, surface tension in the water surrounding a bubble keeps the water from breaking up a bubble into smaller bubbles once a bubble is sufficiently small.
     
  14. Oct 2, 2018 #13

    DaveC426913

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    And to add either confusion or illumination to the mix:

    Air bubbles are only spherical when they are small enough that surface tension can force then into that shape.

    Larger bubbles have a flat(ish) floor.
    main-qimg-41d13693ae44fe4f479b5fb2511a652e.png
    240_F_108697604_H32segENldE9wJYlB2Fv9eWFaXj4lHes.jpg

    This may make it easier to see how the water is attempting to flow downward around and under the bubble.
     
  15. Oct 3, 2018 #14

    sophiecentaur

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    The idea about the shape of big bubbles is similar to the notion that raindrops are ‘tear shaped’. In both cases the motion through a medium distorts them and will lead to a flattened front and wobbly edge.
     
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